Acoustic impedance

Specific acoustic impedance is the product of density and sound velocity, Z = ρ · c. It is the material property that decides how much sound crosses an interface and how much comes straight back. Units are kg·m⁻²·s⁻¹ (rayl); NDT normally uses MRayl (10⁶ rayl), so steel is about 46 MRayl, water 1.5 MRayl, air 0.0004 MRayl.

Impedance is what makes ultrasonics work at all. The mismatch between steel and air is so extreme that a lack of fusion, a lamination or a gas pore reflects essentially all of the incident energy, which is why a hairline air gap is a perfect reflector while a tight, metallurgically bonded interface may be almost invisible. The same mismatch is why couplant is mandatory: without it the probe-to-part interface is a steel/air boundary and nothing gets in.

Impedance is mode-dependent because velocity is. Shear impedance in steel (7850 × 3240 ≈ 25.4 MRayl) is well below compression impedance (≈ 46.3 MRayl); this is one reason shear waves do not couple through a liquid layer — liquids support no shear stress at all.

Use the value with the interface reflection/transmission calculation to work out the loss at a wedge-to-steel boundary, at a clad interface, or at a water path in immersion testing.

Z = ρ · c
Z [MRayl] = ρ [kg/m³] × c [m/s] ÷ 10⁶